fix: correct Phase 3 implementation and remove invalid OCR track alignment

Address Phase 3 accuracy issues identified in review:

**Issue 1: Invalid OCR Track Alignment Code**
- Removed alignment extraction from region style (lines 1179-1185)
- Removed alignment-based positioning logic (lines 1215-1240)
- Problem: OCR track has no StyleInfo (extracted from images without style data)
- Result: Alignment code was non-functional, always defaulted to left
- Solution: Simplified to explicit left-aligned rendering for OCR track

**Issue 2: Misleading Task Completion Markers**
- Updated 5.1: Clarified both tracks support line-by-line rendering
  - Direct: _draw_text_element_direct (lines 1549-1693)
  - OCR: draw_text_region (lines 1113-1270, simplified)
- Updated 5.2: Marked as "Direct track only"
  - spacing_before: Applied (adjusts Y position)
  - spacing_after: Implicit in bbox-based layout (recorded for analysis)
  - indent/first_line_indent: Direct track only
  - OCR: No paragraph handling
- Updated 5.3: Marked as "Direct track only"
  - Direct: Supports left/right/center/justify alignment
  - OCR: Left-aligned only (no StyleInfo available)

**Technical Clarifications**
- spacing_after cannot be "applied" in bbox-based layout
- It is already reflected in element positions (bbox spacing)
- bbox_bottom_margin shows the implicit spacing_after value
- OCR track uses simplified rendering (design decision per design.md)

**Modified Files**
- backend/app/services/pdf_generator_service.py
  - Removed lines 1179-1185: Invalid alignment extraction
  - Removed lines 1215-1240: Invalid alignment logic
  - Added comments clarifying OCR track limitations
- openspec/changes/pdf-layout-restoration/tasks.md
  - Added "(Direct track only)" markers to 5.2 and 5.3
  - Changed 5.3.5 from "Add OCR track alignment support" to "OCR track: left-aligned only"
  - Added 5.2.6 to note OCR has no paragraph handling

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/app/services/pdf_generator_service.py

@@ -1172,18 +1172,11 @@ class PDFGeneratorService:
             logger.info(f"[文字] '{text[:30]}' → PDF位置: ({pdf_x:.1f}, {pdf_y:.1f}), 字體:{font_size:.1f}pt, 寬x高:{bbox_width:.0f}x{bbox_height:.0f}")
 
             # Set font with track-specific styling
+            # Note: OCR track has no StyleInfo (extracted from images), so no advanced formatting
             style_info = region.get('style')
             is_direct_track = (self.current_processing_track == 'direct' or
                               self.current_processing_track == ProcessingTrack.DIRECT)
 
-            # Extract alignment from style (if available)
-            alignment = 'left'  # Default alignment
-            if style_info:
-                if hasattr(style_info, 'alignment') and style_info.alignment:
-                    alignment = style_info.alignment
-                elif isinstance(style_info, dict) and 'alignment' in style_info:
-                    alignment = style_info['alignment']
-
             if style_info and is_direct_track:
                 # Direct track: Apply rich styling from StyleInfo
                 self._apply_text_style(pdf_canvas, style_info, default_size=font_size)
@@ -1197,10 +1190,11 @@ class PDFGeneratorService:
                 pdf_canvas.setFont(font_name, font_size)
 
             # Handle line breaks (split text by newlines)
+            # OCR track: simple left-aligned rendering
             lines = text.split('\n')
             line_height = font_size * 1.2  # 120% of font size for line spacing
 
-            # Draw each line with alignment
+            # Draw each line (left-aligned for OCR track)
             for i, line in enumerate(lines):
                 if not line.strip():
                     continue  # Skip empty lines
@@ -1217,37 +1211,9 @@ class PDFGeneratorService:
                     current_font_size = font_size * scale_factor * 0.95  # 95% to add small margin
                     current_font_size = max(current_font_size, 3)  # Minimum 3pt
                     pdf_canvas.setFont(font_name, current_font_size)
-                    text_width = pdf_canvas.stringWidth(line, font_name, current_font_size)
 
-                # Calculate X position based on alignment
-                line_x = pdf_x
-                if alignment == 'center':
-                    line_x = pdf_x + (bbox_width - text_width) / 2
-                elif alignment == 'right':
-                    line_x = pdf_x + bbox_width - text_width
-                elif alignment == 'justify' and i < len(lines) - 1:
-                    # Justify: distribute extra space between words (except last line)
-                    words = line.split()
-                    if len(words) > 1:
-                        total_word_width = sum(pdf_canvas.stringWidth(word, font_name, current_font_size) for word in words)
-                        extra_space = bbox_width - total_word_width
-                        word_spacing = extra_space / (len(words) - 1)
-
-                        # Draw words with calculated spacing
-                        x_pos = pdf_x
-                        for word in words:
-                            pdf_canvas.drawString(x_pos, line_y, word)
-                            word_width = pdf_canvas.stringWidth(word, font_name, current_font_size)
-                            x_pos += word_width + word_spacing
-
-                        # Reset font and skip normal drawString
-                        if text_width > bbox_width:
-                            pdf_canvas.setFont(font_name, font_size)
-                        continue
-                # else: left alignment uses line_x = pdf_x
-
-                # Draw text at calculated position
-                pdf_canvas.drawString(line_x, line_y, line)
+                # Draw text at left-aligned position (OCR track uses simple left alignment)
+                pdf_canvas.drawString(pdf_x, line_y, line)
 
                 # Reset font size for next line
                 if text_width > bbox_width: